Abstract: Selecting Guanzhong City Group,Shaanxi,China as the study area,based on the atmospheric visibility, precipitation,relative humidity,wind speed and weather phenomenon data of 8 climate stations in 3 times of a day during 1981 to 2013,and using the statistical method and partial accumulative anomaly method,this paper discussed the ann-ual,seasonal and daily change law of low visibility events,analyzed the partial multiple periods, less time,temp-oral and spatial distribution features of low visibility weather,and researched the influence of meteorological and environmental factors on low visibility events in Guanzhong area. The results show as follows:( 1)The multi-year aver-age visibility from 1981 to 2013 in Guanzhong area is about 14.1 km and the average annual visibility showed decreasing tendency from 1981 to 2013. The average daily visibility is mainly between 10-20 km,accounting for about 54.6% of the total events. The multi-year average low visibility events is about 110.9 d with the least occu-rrence in June and the most in December,and the multi-year average low visibility events in winter was significa-ntly higher than that in summer. Low visibility is mostly generated in night and early morning. The percentage of low visibility events persisting for more than 2 days is about 83.2%,while for single day,it is only 16.8%.(2)The low visibility events occurred most in Baoji with 139.2d/a,followed by that in Hancheng with 63.2d/a. The diffe-rence between low visibility days in east and west cities has significantly reduced. (3) Frog is the main weather phenomenon that affected the atmospheric visibility,followed by precipitation and haze. Low visibility events related to frog accounted for 91.8% of the total events,and therein,the low visibility events caused by frog and haze together accounted for 26.3%,which occurred mainly in winter,and most in January and December. Besides,low visibility events caused by blowing sand and sand dust storm occurred mainly in the spring.(4)Relative humidity and wind speed also are the important meteorological influencing factors. Low visibility events occur most frequently when daily average relative humidity was between 80% and 90%,and the probability was about 33.3%. When average daily relative humidity was equal to or greater than 70%,the occurrence of low visibility events accounted for more than 75% of the total events. The low visibility events occurred most under static and light wind,which is more than 80% of the total events. In addition,the mass concentrations of SO₂,NO₂ and PM₁₀ in low visibility events are respectively 52%,32% and 39% higher than that in good visibility events. This research can provide a necessary theoretical basis for accurately forecasting the low visibility events,and provide scientific references for relevant departments to release early warning information and take effective prevention measures for low visibility events.

Key words: visibility, low visibility events, Guanzhong area, temporal and spatial change, influence factors